This invention relates to a method and apparatus useful for testing powders and other particulate bulk solids with the object of evaluating their segregation tendencies when fluidized under industrial conditions.
To facilitate the industrial processing of powders and other particulate solids it is common practice to blow air or other gas into the body of the material. At a sufficient gas flow rate this results in fluidization, a state in which the solids exhibit fluid-like properties. Fluidization may also occur unintentionally, for example during transfer of powder from a blender to a bin, when air flow through the material may fluidize or partially fluidize the material.
However, fluidization is the driving force for a segregation mechanism that alters the uniformity of the properties of the solids in different parts of the body, notably the particle size distribution, but also other properties including for example particle shape, chemical assay, bulk density, color, and solubility. Nonuniformity of these properties generally degrades the quality of the industrial product.
To obtain useful data on the tendency of a particular body of solids to undergo significant segregation when fluidized under industrial conditions, simulation test practices have been devised. These simulate the industrial conditions using a quantity of the solids in a test chamber, employing an accurately repeatable fluidization test procedure. At the conclusion of a test, multiple samples are taken from different parts of the test chamber and subjected to separate assays and analyses of the properties of interest. These results for a new material may be compared with the results for other solids having known segregation properties when similarly tested in the same apparatus and with the same fluidizing test procedure. This comparison provides an indication of potential of the new material to segregate by fluidization in a given industrial application.
The tester can also be used simply to determine the suitability of a new material to fluidize in an industrial application, irrespective of segregation potential or concerns.
The objects of the present invention are directed to improved test apparatus and procedures. The resulting samples may be evaluated using presently existing or future techniques.
A standard practice for measuring fluidization segregation tendencies of powders is described by ASTM International under Designation D6941-03. A suitable apparatus for the purpose is described in U.S. Pat. No. 6,487,921. A vertical columnar test chamber is filled with a bed of the solids, subjected to a fluidization procedure and adapted for removal of samples from several vertical levels of the chamber after completion of the procedure. During the procedure air or other gas is forced into the bottom of the chamber under pressure, and the gas flow rate is measured and increased to a “high flow rate” at which the bed is observed to be fluidized. The flow rate is then reduced to a “low flow rate” at which a minimum level of fluidization is noted, then held at or near the low flow rate for a predetermined “hold time,” and finally reduced to zero. The fluidization of the bed allows or may cause segregation of the material. For example, in powders of mixed particle sizes, segregation causes the lighter, finer particles to increase in concentration towards the top, and the coarser, heavier particles to increase in concentration at the bottom.
In practice, the existing standard practice and its variations have a number of drawbacks. For example, it is frequently necessary to run two tests for each material, a first or characterizing test to determine the appropriate high and low flow rates, and a second or actual test to produce the samples for analysis.
The existing test procedure is influenced by subjective factors varying with the individual performing the test, including observation of the material behavior such as formation of bubbles or turbulence at various levels in the material and expansion or lifting of the bed of material, and control of the rate of change in the flow rate. Thus different persons can produce different test conditions and cause inaccurate, nonrepeatable test results.
The existing standard test procedure typically yields individual samples of substantial volume which, although suitable for certain types of analysis, are often too large for some other common analytical methods. As a result, time consuming sub-sampling, sample splitting or riffling procedures are needed to obtain validly representative smaller quantities for analysis. Sub-sampling can also lead to errors and material loss.
The existing standard test has proven ineffective for certain materials that do not fluidize easily using the prescribed time and flow rate profile.
The existing standard test procedure typically consumes as much as 85 ml of material not only for the first or characterizing test, but also another 85 ml of material for the second or actual test. This is often more material than the quantity available during early stages of development of a particulate solid material, for example a pharmaceutical formulation. Attempts to reduce the size of the current test chamber, while employing the same fluidization profile described in the ASTM method, are ineffective for cohesive materials, since the wall effects, i.e. the total friction along the walls of the chamber relative to the weight of the material, are more pronounced.